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OS .91fsr question
I just purchased a new/old OS .91fsr that came across the RCU classifieds, what a beauty and a beast, it is the size of a 120AX, maybe a little larger. Have any of you run one of these before? What would be a good size prop to try? This engine has a large carb, large cylinder ports and passages, large everything. From the looks of it, I would expect it to breathe and rev better than a .91fx maybe with smaller props? Anyone know? Does anyone know what was changed with the .90fsr (circa 1979) to prompt OS to introduce the .91fsr (1985) a few years later? I am curious, can't find much info on these engines. Mike |
RE: OS .91fsr question
It could be due to noise regulations. The 91 may be better at lower rpm if so, or it may be a higher performance model for the competition crowd? Either way I would get a range of props and do some testing.
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RE: OS .91fsr question
These are user friendly and long lasting engines that are heavy and make average power. The 90 came before the 108 and had a different looking case but is nearly identical dimensionally. The 91 shares the same case and head casting with the 108 with different identification. All 90/91 parts are interchangeable with the only difference being the case and head castings. Contrary to popular belief the 90/91FSR have a 61 size crank with an even smaller gas passage. Only the marine version could rev. The carb is same between 90 and 108 making the smaller engine require more rpm and nitro for good fuel draw. Prop for 11k rpm, a 14x5/6 worked well for slow planes and a 13x7/8 for sport planes.
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RE: OS .91fsr question
You are right the carb is the 7D with a sleeve in the venturi area, same size as my 1.08. With just my eye, the crankshaft gas passage does not look to be that small. I will measure the I.D. and post back later. The carb throat is pretty large for a 15cc engine, I may swap it for a 7L to compare RPMs and fuel draw when I put this engine into service. |
RE: OS .91fsr question
ORIGINAL: Kweasel These are user friendly and long lasting engines that are heavy and make average power. The 90 came before the 108 and had a different looking case but is nearly identical dimensionally. The 91 shares the same case and head casting with the 108 with different identification. All 90/91 parts are interchangeable with the only difference being the case and head castings. Contrary to popular belief the 90/91FSR have a 61 size crank with an even smaller gas passage. Only the marine version could rev. The carb is same between 90 and 108 making the smaller engine require more rpm and nitro for good fuel draw. Prop for 11k rpm, a 14x5/6 worked well for slow planes and a 13x7/8 for sport planes. This is a fairly accurate assement of the 90 FSR. I still have mine in good running order and 14 inch props were the norm. But to be fair that is usual for ST 90 and the Webra 90 of that time. The 90's just did not shine. Good runneers but not stellar. Dennis |
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